This invention relates to a process for the separation of arsenic acid from a sulfuric acid-containing solution. More particularly, this invention provides a process for the separation of arsenic acid from a sulfuric acid-containing solution wherein the arsenic acid is converted to copper arsenate which can be used as a raw material for the manufacture of wood preservatives.
In some manufacturing processes, waste mixtures of sulfuric acid, water, and arsenic acid result. It would be advantageous to have a process wherein arsenic acid in such waste mixtures is transformed into a useful product and any residual arsenic acid in the resulting solution is removed.
As such, the need exists in the art for a process for the transformation of arsenic acid in such waste mixtures of arsenic and sulfuric acids and water to a useful product and the removal of any residual arsenic acid in the resulting solution.
Methods exist in the art for generating useful products from arsenic acid. For example, methods for the preparation of copper arsenates from arsenic acid wherein sulfuric acid is not part of the reaction medium are taught by U.S. Pat. Nos. 4,103,000; 4,804,494; 4,959,203; and 5,002,748; and Mirza et al., "Formation and Stability Studies of Iron-Arsenic and Copper-Arsenic Compounds from Copper Electrorefining Sludge", Arsenic Metall. Fundam. Appl. Proc. Symp. 37 (1988).
U.S. Pat. No. 4,103,000 teaches the preparation of copper arsenate slurries by reacting copper metal and an oxidizing agent. The catalyst is a strong acid such as HNO.sub.3, HCl, and H.sub.2 SO.sub.4 and oxygen gas is used as the oxidizing agent. The method requires long reaction times of four to 10 hours.
U.S. Pat. No. 4,804,494 teaches the preparation of copper arsenate by reacting copper metal with a solution of sodium arsenate, a solution or suspension of arsenic trioxide of at least 50 percent by weight concentration in the presence of hydrogen peroxide having a concentration of 50 to 75 percent by weight.
U.S. Pat. No. 4,959,203 teaches the preparation of copper arsenate by reaction of a solution of sodium arsenate with copper sulfate to first form a copper and arsenic-containing solution without precipitating copper arsenate by controlling the pH to precipitate the impurities. After removal of the impurities, the solution is neutralized to precipitate copper arsenate. See also U.S. Pat. No. 5,002,748.
Mirza et al., supra, teach the formation of copper arsenates at room temperature by the reaction of arsenic acid in a concentration of ten grams per liter of arsenic solution with copper in a concentration of thirty grams per liter of solution at a pH of 4.5 at room temperature. At low temperature, copper arsenate, Cu.sub.3 (AsO.sub.4).sub.2, is formed and at elevated temperature, copper hydroxy arsenate, Cu(OH)CuAsO.sub.4 is formed.
A method for the preparation of copper arsenates from a solution formed from mixing a copper sulfate/sulfuric acid solution with an arsenic acid solution is taught by U.S. Pat. No. 4,961,909. The reference teaches that an arsenic-containing material is subjected to an oxidative pressure leach with sodium hydroxide forming a leach solution containing sodium arsenate. The solution of sodium arsenate is treated with copper sulfate to precipitate copper arsenate and give a solution of sodium sulfate. At least one of the sodium arsenate-containing solution and the sodium sulfate solution is subjected to electrolysis with membranes for at least partial conversion to arsenic acid and sulfuric acid, respectively, and sodium hydroxide. The sulfuric acid generated may be used in the preparation of the copper sulfate.
These references do not address how to remove residual arsenic from the waste mixture after copper arsenate is formed. In an attempt to separate arsenic acid from the waste mixtures of arsenic and sulfuric acids and water, bipolar membranes were used. These membranes did not prove to be useful for this separation because the solutions had to be diluted with water and still, good separation did not occur.
Also in an attempt to separate arsenic acid from the waste mixtures of arsenic and sulfuric acids and water, ion exchange was used. Ion exchange also did not prove to be useful for this separation because the solutions had to be diluted with water so as to not attack the ion exchange resin. Then, acid had to be added to regenerate the ion exchange resin and still, good separation did not occur.
Also in an attempt to separate arsenic acid from the waste mixtures of arsenic and sulfuric acids and water, solvent extraction was used. Solvent extraction also did not prove to be useful because sulfuric acid was co-extracted with the arsenic acid.
Also in an attempt to separate arsenic acid from the waste mixtures of arsenic and sulfuric acids and water, sulfide precipitation which involved hydrogen sulfide or sodium hydrogen sulfide was used. Sulfide precipitation is not advantageous because the sulfide precipitate is sticky and difficult to handle.
Also in an attempt to separate arsenic acid from the waste mixtures of arsenic and sulfuric acids and water, the addition of titanium to precipitate titanium arsenate was used. Titanium arsenate precipitation did not prove to be useful because only 70% of the arsenic is precipitated and then the titanium arsenate had to be converted to copper arsenate.